1. Field
This invention pertains in general to nuclear power plants and fuel rods assemblies or bundles that are positioned in the nuclear reactor core and, more particularly, to a rib-type roughness placed on the one or more of the fuel rods to provide enhanced heat transfer in the fuel rod assemblies or bundles.
2. Description of Related Art
The reactor core of a nuclear power plant, such as a pressurized water reactor (PWR), contains a plurality, e.g., bundles or assemblies, of nuclear fuel rods. The fuel rods contain uranium oxide fuel. The fuel is encased in sealed tubes, commonly referred to as the fuel cladding. The cladding maintains the fuel in a position, for which controlled fission can proceed and generate heat. The cladding then transfers the heat from the fuel to pressurized water that circulates around the primary loop of the reactor coolant system. The heated water in the primary loop is used to boil water in a steam generator and the steam is then expanded in a turbine that powers an electrical generator.
For the purpose of illustration, FIG. 1 shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel 10 having a closure head 12 enclosing a nuclear core 14. A liquid reactor coolant, such as water, is pumped into the vessel 10 by pump 16 through the core 14 where heat energy is absorbed and is discharged to a heat exchanger 18, typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump 16, completing the primary loop. Typically, a plurality of the above described loops is connected to a single reactor vessel 10 by reactor coolant piping 20.
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies which each has top and bottom nozzles. A plurality of elongated transversely spaced guide thimbles extends longitudinally between the nozzles. A plurality of transverse support grids are axially spaced along and attached to the guide thimbles.
Each fuel assembly is composed of a plurality of elongated fuel elements or rods which are transversely spaced apart from one another and from the guide thimbles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core which is sufficient to support a high rate of nuclear fission, and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
The grids are used to precisely maintain the spacing and support between the fuel rods in the reactor core, provide lateral support for the fuel rods, and induce mixing of the coolant.
The exemplary reactor pressure vessel 10 and nuclear core 14 as shown in FIG. 1 are shown in more detail in FIG. 2. The nuclear core 14 includes a plurality of parallel, vertical, co-extending fuel assemblies 22. For purpose of this description, the other vessel internal structures can be divided into lower internals 24 and upper internals 26. In conventional designs, the lower internals' function is to support, align and guide core components and instrumentation as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assemblies 22 (only two of which are shown for simplicity in FIG. 2), and support and guide instrumentation and components, such as control rods 28. In the exemplary reactor shown in FIG. 2, coolant enters the reactor vessel 10 through one or more inlet nozzles 30, flows down through an annulus between the vessel and the core barrel 32, is turned 180° in a lower plenum 34, passes upwardly through a lower support plate 37 and a lower core plate 36 upon which the fuel assemblies are seated and through and about the assemblies. In some designs, the lower support plate 37 and the lower core plate 36 are replaced by a single structure, a lower core support plate having the same elevation as 37. The coolant flow through the core and surrounding area 38 is typically large on the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and frictional forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals, including a circular upper core plate 40. Coolant exiting the core 14 flows along the underside of the upper core plate 40 and upwardly through a plurality of perforations 42. The coolant then flows upwardly and radially outward to one or more outlet nozzles 44.
One of the exemplary fuel assemblies 22 as shown in FIG. 2 is shown in more detail in FIG. 3. Each of the fuel assemblies 22 includes fuel rods 66 grouped in an array thereof. The fuel rods 66 are held in spaced relationship with one another by the grids 64 spaced along the fuel assembly length. Each fuel rod 66 includes a plurality of nuclear fuel pellets 70 and is closed at its opposite ends by upper and lower end plugs 72 and 74, respectively. The pellets 70 are maintained in a stack by a plenum spring 76 disposed between the upper end plug 72 and the top of the pellet stack. The fuel pellets 70, composed of fissile material, are responsible for creating the reactive power of the reactor. There is a cladding which surrounds the pellets to function as a barrier to prevent the fission byproducts from entering the coolant and further contaminating the reactor system.
The grids 64 also provide for coolant mixing to decrease the maximum coolant temperature. Since the heat generated by each of the fuel rods 66 is not uniform, there are thermal gradients in the coolant. One important parameter in the design of the fuel assemblies 22 is to maintain the efficient heat transfer from the fuel rods 66 to the coolant. The higher the amount of heat removed per unit time, the higher the power being generated.
At high enough coolant temperatures, the rate of heat that can be removed per unit of cladding area in a given time decreases abruptly in a significant way. This phenomenon is known as deviation from nucleate boiling or DNB. If within the parameters of reactor operation, the coolant temperature would reach the point of DNB, the cladding surface temperature would increase rapidly in order to evacuate the heat generated inside the fuel rod and rapid cladding oxidation would lead to cladding failure. It is clear that DNB needs to be avoided to prevent fuel rod failures. Since DNB, if it occurs, takes place at the point where the coolant is at its maximum temperature, it follows that decreasing the maximum coolant temperature by coolant mixing within the assembly permits the generation of larger amounts of power without reaching DNB conditions.
Normally, improved mixing is achieved by the use of mixing vanes in the down flow side of the grid 64 structure. The effectiveness of mixing is dependent upon the shape, size and location of the mixing vanes relative to the fuel rods 66.
As shown in FIG. 4, mixing vanes 89 are installed on the upper surface of one of the plurality of grids 64. The mixing vanes 89 create turbulence in the coolant in the nuclear core 14 (shown in FIG. 2), which promotes the transfer of heat from the fuel rod cladding to the coolant. This turbulence is locally intense and rapidly dissipates in the region located downstream of the mixing vanes 89, e.g., in the span or spacing between consecutive grids 64. Thus, for example, in the region 90 the mixing vanes 89 operate to create turbulence and promote heat transfer. However, beyond region 90 further downstream of each of the grids 64, the turbulence dissipates and the heat transfer is lower, resulting in increased limiting conditions for cladding temperature, crud deposition, and oxidation.
It is, therefore, an object of this invention to provide a modified fuel rod cladding having a particular rib-type roughness at least partially formed on the exterior cladding surface that will enhance the heat transfer of the fuel rod in a nuclear reactor and, particularly, in the region downstream of the mixing vanes wherein turbulence is dissipated.